Optical beam power controllers are useful in a wide variety of practical and experimental applications such as optical communications, optical instrumentation and optical inspection systems. Typically the beam is a linearly polarized laser beam.
A wide variety of techniques have been used to control beam power, but none provide fast, continuous control with low loss, small beam displacement and minimal group-velocity dispersion. Traditional methods include translation of neutral density filters with spatially varying optical density. They also include acousto-optical, electro-optical, and liquid-crystal modulation as well as variation of Fresnel reflections and the rotation of half-wave plates. The methods vary widely in their speed, dynamic range, effects on beam-pointing, wavelength sensitivity, dispersion, and cost.
Electro-optical and acousto-optical methods are by far the fastest (sub-.mu.s). They are widely employed where speed is essential such as in pulse-pickers or in fluorescence lifetime instrumentation. Unfortunately they are expensive.
Acousto-optical modulators (Bragg cells) have a large dynamic range but high insertion losses (10-20%). They also have angular wavelength dispersion in the diffracted beam which complicates use with very short laser pulses. The fundamental (undiffracted) beam affords, at best, a 10:1 intensity modulation.
Electro-optical modulators (Pockels-cells) require high voltages or small apertures, which complicates alignment, in conjunction with a long optical pathlength, which causes group-velocity dispersion.
Methods that involve the mechanical movement of optical elements are slower than acousto-optical and electro-optical techniques but are generally more cost effective. Variable neutral density (ND) filters suffer from intensity variations across the beam, laser power dependent thermal-lensing effects, and beam pointing variations. Rotation of half-wave plates suffers from pointing variations if the plate surfaces are not perfectly parallel. Both variable ND filters and rotating half-wave plates require typically tens of milliseconds to change the intensity. Accordingly there is a need for an inexpensive beam power controller providing fast, continuous control.